Conventionally, curable resins such as epoxy resins have been widely used in fields such as molding materials, and materials for laminates and adhesives. Because rapid curability of these curable resins is desirable from the viewpoint of improving productivity, compounds that accelerate the curing reaction, namely curing accelerators, are commonly added to the curable resin compositions. For example, in the technical field associated with the encapsulating of elements of electronic parts such as transistors and ICs, of the various possible curable resins, compositions that use an epoxy resin as the base resin are particularly widely used. The reason for this widespread use is that epoxy resins offer a favorable balance over a wide range of properties, including moldability, electrical properties, moisture resistance, heat resistance, mechanical properties, and the level of adhesion to inserts. Combinations of an ortho-cresol novolac epoxy resin and a phenol novolac curing agent offer a particularly superior balance of the above properties, and are consequently the most widely used base resins for molding materials used for IC encapsulation. Curing accelerators including amine compounds such as tertiary amines or imidazoles, and phosphorus compounds such as phosphines or phosphonium compounds are typically used in these epoxy resin compositions.
However in recent years, in the field of encapsulating elements of electronic parts, there is a growing trend towards higher density packaging of electronic parts on printed wiring boards, and accompanying this trend, surface-mounted packages are gradually becoming more common than conventional pin insertion-type packages. However, compared with pin insertion-type packages, surface-mounted packages tend to exhibit inferior resistance to package cracking during soldering, so-called reflow crack resistance. In other words, in surface mounted ICs such as IC and LSI, the element-occupied volume within the package is being gradually increased in order to increase the packaging density, meaning the thickness of the package wall is becoming extremely thin. Moreover, surface-mounted packages are exposed to more severe conditions during soldering than pin insertion-type packages.
Specifically, in the case of a pin insertion-type package, the pins are inserted into the wiring board, and soldering is then conducted from the underside of the wiring board, and consequently the package is not exposed directly to high temperatures. In contrast, in the case of a surface-mounted IC, the IC is temporarily secured to the surface of the wiring board, and treatment is then conducted in a solder bath or a reflow apparatus, meaning the package is exposed directly to the high soldering temperature. As a result, if the IC package has absorbed moisture, then this absorbed moisture expands rapidly during soldering, and may cause cracks within the package, which represents a significant problem during package molding.
As a result of these circumstances, in order to improve the reflow crack resistance of surface-mounted packages, epoxy resin compositions containing increased quantities of inorganic fillers have been reported. However, as the quantity of inorganic fillers is increased, the fluidity of the resin composition deteriorates, which often leads to a deterioration in the performance of the package, including molding problems such as the generation of filling defects or voids upon molding, or conduction faults caused by breakage of the IC chip bonding wires. Accordingly, there has been a limit to how far the quantity of inorganic fillers can be increased, meaning achieving significant improvements in the reflow crack resistance has been difficult. Particularly in those cases where a phosphorus-based curing accelerator such as triphenylphosphine or an amine-based curing accelerator such as 1,8-diazabicyclo[5.4.0]undecene-7 is added to the above type of epoxy resin in order to improve the curing rate, a significant deterioration in the fluidity of the resin composition tends to be caused. As a result, in addition to improvements in the reflow crack resistance of packages, improvements in the fluidity of resin compositions is also currently sought.
In order to improve the fluidity of epoxy resin compositions comprising a high proportion of an inorganic filler, Japanese Patent Laid-Open No. H09-157497 proposes a method that uses the addition reaction product of triphenylphosphine and 1,4-benzoquinone as a curing accelerator. In other methods, Japanese Patent Laid-Open No. 2004-156035 and Japanese Patent Laid-Open No. 2004-156036 propose methods that use phosphoniophenolates as the curing accelerator.